Electrodeposited copper foil for PCB having barrier layer of Zn-Co-As alloy and surface treatment method of the copper foil

ABSTRACT

An electrodeposited copper foil to be laminated on an insulation substrate for printed circuit boards, comprises a barrier layer formed of ternary alloy of Zn—Co—As on the copper foil. According to another aspect of the invention a surface treatment method of an electrodeposited copper foil for printed circuit boards is performed comprises the steps of electrolytically treating the copper foil in the electrolytic solution containing pyrophosphoric acid potassium of 10˜200 g/l, Zn of 0.1˜20 g/l, Co of 0.1˜20 g/l and As of 0.05˜5 g/l. Further, the electrolytic solution remains at the temperature of 20˜50° C. and pH=9˜13, and the copper foil is electrolytically treated for 2˜20 seconds at the cathode current density of 0.5˜20 A/dm 2 .

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an electrodeposited copper foil laminated on an insulation substrate for printed circuit boards and a surface treatment method thereof, and more particularly to an electrodeposited copper foil for printed circuit boards and a surface treatment method in which a barrier layer of alloy of Zn—Co—As is formed on the electrodeposited copper foil so that adhesive strength to the insulation substrate, heat-resistance, and anticorrosion are improved, and in which innoxiousness and anticorrosion of equipment can be assured by using pyrophosphoric acid potassium as a complexing agent in the electrolytic process.

[0003] 2. Description of the Related Art

[0004] Generally, printed circuit boards are widely used in a variety of electronic applications, such as radios, televisions, washing machines, VCR, computers, etc., and precise controllers for industrial electronic devices. Recently, printed circuit boards are developed to meet the demand for fineness, miniaturization and high density of integration of the electronic articles, which requires high quality of the printed circuit board.

[0005] As insulation substrates for industrial printed circuits, in general, a flame-retardant prepreg is used which is made of epoxy resin and glass fibers impregnated therein. The printed circuit board is fabricated by laminating the electrodeposited copper foil on the insulation substrate at high pressure and temperature, and then etching the copper to the circuit pattern.

[0006] Raw materials for the electrodeposited copper foil are formed by consecutive electrolytic deposition in copper sulfate solution. The surface of the raw foil is nodularized for better adhesion with respect to the insulation substrate. Consequently, the electrodeposited copper foil is obtained by electrodepositing a barrier layer on the nodularized surface, and then treating the barrier layer with electrolytic chromate anticorrosion process.

[0007] However, brown stains and spottings are frequently appeared through out the resinous layer of the finished printed circuit board, which tend to adversely affect the insulation properties of the resin and consequently the performance of the printed circuits so that the physical properties of the final product is undesirable.

[0008] The actual mechanism for the brown stain is not fully understood. However, the cause appears to be the result of chemical and mechanical interaction between the copper foil and the resinous layer. Such a drawback may cause degradation of the adhesion between the copper foil and the insulation substrate in the process of lamination that involves high pressure and high temperature treatment.

[0009] To solve the above problem, U.S. Pat. Nos. 3,585,010 and 4,049,481 disclose surface treatment method of copper foil by which the brown stains are removed during the adhesion of the insulation substrate and the copper foil, and the copper foil having excellent adhesion can be obtained. Also, Korean Pat. No.065333 in 1993 provides a method in which alloy of Cu—As is coated using pulse current to thereby improve the adhesion to the insulation substrate.

[0010] According to the conventional methods to obtain the electrodeposited copper foil, copper oxides on the surface of the raw foil to be laminated with the insulation substrate are removed to activate the surface. Then, the surface is treated to provide a modularized surface with copper, and a barrier layer is electrolytically formed on the surface. Finally, the surface is processed by anticorrosive treatment. Metal used for the barrier layer comprise nickel, tin, zinc, cadmium, arsenic and the alloy thereof. It is known that the above metal and alloy prevent stains at the interface of the adhesion, enable heat-resistant adhesion between the insulation substrate resin and the copper foil, and facilitate etching of the copper foil. Most electrolytic precipitation of metal is conducted in the solution of metal complexing compound rather than plain metal ions. Also, the electrolytic deposition formed in the bath of metal complexing compound exhibits excellent electrodeposition property so that uniform electrodeposition can be made to deep grooves as well as nodules on the surface to become minute, uniform and brilliant deposited film. Further the electrolytic deposition enables electrochemical coating with alloy of base metal and noble metal.

[0011] However, the product by the conventional method has inferior properties in heat-resistance and anticorrosion, particularly, in the process of copper clad laminating and the manufacture of the printed circuit board. Also, the surface treatment of the barrier layer using brass may cause ‘undercutting’ phenomenon in which the adhesive area between the copper foil circuit and the insulation substrate layer decreases because zinc has rapid corrosion tendency compared to copper in the solution of copper chloride (I), iron chloride (II) or ammonium persulfate during the formation of circuits, which leads to the separation of copper foil circuit from the insulation substrate. In the above conventional method, the complexing agent in the electrolytic solution to form the alloy layer during the formation of barrier layer comprises cyanogen CN, sodium citrate Na₃C₆H₅O₇, and tartaric acid.

[0012] However, the electrolytic solution containing cyanogen as the complexing agent is excellent in plating but toxic solution that may incur poisonous gases during the manufacture and environmental pollution. Further, the solution containing sodium citrate is acid resulting in corrosion of equipment, and tartaric acid is unstable particularly in the plating process of alloy metal having high ionization tendency.

SUMMARY OF THE INVENTION

[0013] The present invention is designed to overcome such disadvantages of the prior art, and an object of the invention is to provide a surface treatment method to obtain electrodeposited copper foils having excellent heat-resistance and anticorrosion by forming a barrier layer of alloy of Zn—Co—As to thereby increase adhesive strength with respect to an insulation substrate. Also, according to the present invention, since electrolytic solution containing pyrophosphoric acid potassium K₄P₂O₇ as a complexing agent is used, poisonous process and corrosion of equipment can be prevented.

[0014] In order to accomplish the above object, the present invention provides an electrodeposited copper foil to be laminated on an insulation substrate for printed circuit boards, which comprises a barrier layer formed of ternary alloy of Zn—Co—As on the copper foil.

[0015] The thickness of the barrier layer is 0.01˜0.2 μm.

[0016] According to another aspect of the present invention, the copper foil is electrolytically treated in the electrolytic solution containing Zn of 0.1˜20 g/l, Co of 0.1˜20 g/l, As of 0.05˜5 g/l, and pyrophosphoric acid potassium of 10˜200 g/l, respectively.

[0017] Furthermore, the electrolytic solution remains at the temperature of 20˜50° C. and pH=9˜13. Preferably, the copper foil is electrolytically treated for 2˜20 seconds at the cathode current density of 0.5˜20 A/dm².

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings. In the drawings:

[0019]FIG. 1 is a flow chart showing a surface treatment method according to a preferred embodiment of the present invention; and

[0020]FIG. 2 is a sectional view of an electrodeposited copper foil according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0021] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIGS. 1 and 2 show a surface treatment method and an electrodeposited copper foil according to a preferred embodiment of the present invention, respectively.

[0022] A surface treatment method of an electrodeposited copper foil according to the present invention will be described, as follows.

[0023] First, a raw foil 100 to be laminated on an insulation substrate is formed in the solution of copper sulfate by a successive electrodeposition process in step S100. In step S200, the surface of the raw foil to be treated is activated by removing copper oxides thereon. Then, the surface is nodularized for improvement of physical adhesion with the insulation substrate to thereby form a layer of nodule 200 on the raw copper foil in step S300. A barrier layer 300 is formed by coating the nodularized surface of the foil with alloy to enhance chemical adhesion in step S400. Finally, electrolytic chromate anticorrosion treatment is carried out with respect to the surface so that the electrodeposited copper foil for the printed circuit board on which a layer of chromate 400 is formed can be obtained.

[0024] According to the preferred embodiment of the present invention, in the step S400, the barrier layer coated on the surface is made of ternary alloy of Zn—Co—As. Thus, quaternary alloy of Cu—Zn—Co—As is formed between the electrodeposited copper foil and the insulation substrate layer after forming the insulation substrate at high pressure and temperature.

[0025] Zinc and cobalt prevent copper ions from diffusing into the insulation substrate during the formation with the insulation substrate at high temperature to thereby decrease the degradation of the insulation substrate and maintain the insulation property thereof. At this time, the concentration of the electrolytic solution for both of zinc and cobalt is preferably in the range of 0.1-20 g/l. It takes long time for process at less than 0.1 g/l, which penalizes full productivity and decreases current efficiency. On the contrary, at the concentration more than 20 g/l, even though the plating can be made, it requires a lot of consumption of pyrophosphoric acid potassium as the complexing agent, which is not preferable in terms of yield expenses.

[0026] Arsenic restrains escape of zinc in the alloy layer and the undercutting phenomenon even when using chemical agents such as hydrochloric acid. In addition, arsenic improves the heat-resistance together with zinc during the formation with the insulation substrate at high temperature. At this time, the concentration of the electrolytic solution for arsenic preferably lies in the range of 0.05˜5 g/l. It takes long time for process at less than 0.05 g/l, which is not enough for full-alloy. Contrarily, at the concentration more than 5 g/l, it results in excess of arsenic in the alloy layer and a lot of consumption of pyrophosphoric acid potassium as the complexing agent, which is not preferable as the same reason as the above.

[0027] Preferably, the temperature of the electrolytic solution is 20˜50° C. At a low temperature, it is difficult for metallic ions to dissolve into the solution. At a high temperature, the coated surface is not uniform because the control of solution is uneasy. The electrolytic solution preferably remains alkaline at pH=9˜13.

[0028] In addition, the thickness of the alloy coating layer is preferably in the range of 0.01˜0.2 μm. As the alloy coating layer becomes thin, there is little effect of alloy. Otherwise, thick as the alloy coating layer is, it takes long time for etching in the process of the printed circuit boards. The thickness of the alloy coating film is controlled by means of treatment time and cathode current density. Preferably, the treatment time and the cathode current density may be set to 2˜20 seconds and 0.5˜20 A/dm², respectively.

[0029] By the way, using pyrophosphoric acid potassium as a complexing agent results in combination of pyrophosphoric and metallic ions such as zinc, cobalt and arsenic to thereby form a complex compound, which enables to electrochemically minimize the gap of an ionization tendency between zinc having relatively high one and arsenic having relatively low one so that alloy of Zn—Co—As can be obtained.

[0030] Here, the amount of pyrophosphoric acid potassium in the electrolytic solution shall be more than that of metallic ions, preferably to 10˜200 g/l. In the case of the amount less than 10 g/l, metallic ions in the electrolytic solution cannot fully turn to the complex compound but precipitate in the form of metallic hydroxides. Resultantly, the alloy-coating surface is not uniform and the lifespan of plating solution is short due to the increase in HPO₃ ⁻ that is generated in the electrolytic reaction. Also, in the case of the amount more than 20 g/l, even though the plating can be made, it requires a lot of consumption of pyrophosphoric acid potassium, which is not preferable in the aspect of yield expenses.

[0031] Hereinafter, a preferred embodiment of the present invention and advantages thereof will be described compared to the conventional surface treatment methods.

[0032] According to the preferred embodiment of the present invention, a copper foil is treated with Zn—Co—As alloy surface treatment based on the following condition. Composition of solution Zinc sulfate (as Zn) 10 g/l Cobalt sulfate (as Co) 10 g/l Arsenious acid (as As) 1 g/l Pyrophosphoric acid potassium 120 g/l Condition of process Temperature of solution 40° C. pH 11 Cathode current density 10 A/dm² Treatment time 5 seconds

[0033] Zn—Sn surface treatment is accomplished by the conventional surface treatment method as a comparative example, as follows. Composition of solution Zinc sulfate (as Zn) 10 g/l Tin sulfate (as Sn) 10 g/l Arsenious acid (as As) 1 g/l Pyrophosphoric acid potassium 120 g/l Condition of process Temperature of solution 40° C. pH 11 Cathode current density 10 A/dm² Treatment time 5 seconds

[0034] Copper foils treated according to the above methods is press-laminated on the insulation substrate and then etched, respectively. The table shows the bonding strength measured after laminating and etching with respect to the invention and the comparative example. Bonding strength (kgf/cm) After laminating After etching Present Invention 2.5 2.3 Comparative example 2.2 2.0

[0035] As shown in the above table, the substrate of which surface is treated according to the present invention exhibits high adhesive strength after laminating and etching compared to the comparative example.

[0036] According to the electrodeposited copper foil for printed circuit board and the surface treatment method, a barrier layer of alloy of Zn—Co—As formed on the electrodeposited copper foil enhances the bonding strength with respect to the insulation substrate, the heat-resistance and anticorrosion of the electrodeposited copper foil. At the same time, innoxiousness and anticorrosion of equipment can be assured by using the solution containing pyrophosphoric acid potassium as a complexing agent in the electrolytic process.

[0037] The present invention has been described in detail. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 

What is claimed is:
 1. An electrodeposited copper foil to be laminated on an insulation substrate for printed circuit boards, comprising: a barrier layer formed of ternary alloy of Zn—Co—As on the copper foil.
 2. The electrodeposited copper foil according to claim 1, wherein the thickness of the barrier layer is 0.01˜0.2 μm.
 3. A surface treatment method of an electrodeposited copper foil for printed circuit boards, comprising the steps of: electrolytically treating the copper foil in the electrolytic solution containing pyrophosphoric acid potassium of 10˜200 g/l; and forming a barrier layer of ternary alloy of Zn—Co—As on the copper foil.
 4. The surface treatment method of an electrodeposited copper foil according to claim 3, wherein the concentration of Zn, Co and As in the electrolytic solution is 0.1˜20 g/l, 0.1˜20 g/l and 0.05˜5 g/l, respectively.
 5. The surface treatment method of an electrodeposited copper foil according to claim 3, wherein the electrolytic solution remains at the temperature of 20˜50° C. and pH=9˜13.
 6. The surface treatment method of an electrodeposited copper foil according to claim 3, wherein the copper foil is electrolytically treated for 2˜20 seconds at the cathode current density of 0.5˜20 A/dm².
 7. The surface treatment method of an electrodeposited copper foil according to claim 3: wherein the copper foil is electrolytically treated so that the thickness of the barrier layer is 0.01˜0.2 μm. 